1. Field of the Invention
The present invention relates to a surface acoustic wave functional element, and moreover to a suitable thin film piezoelectric substrate thereto and a manufacturing method thereof. The surface acoustic wave functional element is a device utilizing a surface acoustic wave, and includes a filter, a resonator, a convolver or the like.
2. Description of Related Art
LiTaO.sub.3 and LiNbO.sub.3 have very superior properties in electromechanical coupling coefficient, electro-optical effect, non-linear optical effect or the like, and are practically used as a material for a surface acoustic wave (SAW) device. LiTaO.sub.3 and LiNbO.sub.3 have substantially equal material properties including crystal structure, lattice constant, thermal expansion coefficient or the like. A material represented by LiNb.sub.x Ta.sub.1-x O.sub.3 (`x` is 0 or more and 1 or less) exhibits material properties similar to LiTaO.sub.3 and LiNbO.sub.3.
An elastic wave is divided into a bulk wave and a surface acoustic wave. The bulk wave includes longitudinal and shear waves. The surface acoustic wave includes a Rayleigh wave, a Love wave, a Sezawa wave, a pseudo surface acoustic wave or the like. The pseudo surface acoustic wave has a propagation loss. The Rayleigh wave, Love wave, and Sezawa wave do not have such propagation loss. The Love wave is a wave mainly composed of a displacement component vertical to a propagation direction and parallel to a substrate surface. The Rayleigh wave and Sezawa wave are waves that have a less displacement of which the Love wave is mainly composed; and that are mainly composed of a displacement component in a propagation direction, or a depth direction to a substrate surface, respectively. Because the Love wave has shear-wave oriented properties as described above, it has a small propagation loss in a solution. Thus, an application to a surface acoustic wave sensor in the solution or the like is expected.
In creating a high-frequency wide-bandwidth SAW filter, it is preferable to employ a wave having a high surface acoustic wave velocity (V) and a large electromechanical coupling coefficient (K.sup.2) and being free of a propagation loss. In a commercially available single crystal material, V=4000 m/s and K.sup.2 =5.5% are achieved with 128Y LiNbO.sub.3 in a Rayleigh wave which is free of a propagation loss. For use of an SAW device such as SAW filter, assuming that a surface acoustic wave velocity is V and a wavelength of the surface acoustic wave is .lambda., a use frequency is represented by f=V/.lambda.. Hence, in employing SAW filter in a high-frequency band, the low surface acoustic wave velocity results in reduced .lambda.. A pitch of a electrode of interdigital transducer is generally .lambda./4 or less. When .lambda. is reduced, it becomes difficult to carry out an electrode fabrication process using lithography; and therefore, a material having higher V is desired. A technique for improving V includes a method for forming a thin film of LiNbO.sub.3, LiTaO.sub.3, ZnO or the like on a sapphire substrate having a high V and using it, which is practically used with a ZnO thin film. However, the ZnO thin film, has such is a disadvantage that K.sup.2 is small, i.e., not greater than 5%. A material having high K.sup.2 is required to form a filter with a wide bandwidth. In order to obtain a piezoelectric substrate material having high V together with large K.sup.2, there is a growing need for making a thin film of LiTaO.sub.3 and LiNbO.sub.3 having higher V and larger K.sup.2 than ZnO, and a variety of studies have been made. The inventors have succeeded in making a thin film of piezoelectric (012) LiTaO.sub.3 on a (012) sapphire substrate and (001) LiNbO.sub.3 on a (001) sapphire substrate using a laser abrasion method, and has reported that its SAW velocity is significantly higher than that of a bulk material, and this material can be advantageously employed as a material for high frequency. However, the electromechanical coupling coefficients are obtained by theoretical calculation less than 5% for the LiTaO.sub.3 film (Y. Shibata et al., Jpn. J. Appl. Phys., 34 (1995) 249-253.) and less than 6.9% for the LiNbO.sub.3 film (Y. Shibata et al., J. Appl. Phys., 77 (1995) 1489-1503), so their use has been limited.
In a two-layer structure of a substrate and a piezoelectric film, it is known that when a velocity of the longitudinal wave in the substrate is greater than that in a bulk single crystal of a piezoelectric film material, the Sezawa wave or Love wave appears (Y. Shibata et al., Jpn. J. Appl. Phys., 34 (1995) 249-253, T. Mitsuyu et al., J. Appl. Phys., 51 (1980) 2464-2470, etc.). However, a surface acoustic wave functional element employing a Love wave or a Sezawa wave with a large electromechanical coupling coefficient and a high surface acoustic wave velocity is not practically used expect in a case that K.sup.2 is about 4.3% or less at the Sezawa wave in the ZnO film.
Therefore, an object of the present invention is to achieve high V together with large K.sup.2 for a wave free of a propagation loss with a LiNb.sub.x Ta.sub.1-x O.sub.3 film. In particular, another object of the present invention is to generate a Love wave having a high surface acoustic wave velocity and a large electromechanical coupling coefficient.